Republican presidential challenger Mitt Romney has predicted a sleepless night for the White House ahead of a supreme court ruling on Barack Obama's healthcare reform, expected to be delivered on Thursday.

Republicans expect that the justices will rule against some or all of the healthcare package, with the provision for mandatory coverage for almost all Americans looking most vulnerable.

If the supreme court does strikes down the legislation, it would be one of the biggest decisions in its history, and open up a debate about the extent of the politicisation of the court.

Romney, speaking at a campaign event in Sterling, Virginia, said: "My guess is they're not sleeping real well at the White House tonight. That's the way it oughta be."

He added: "There are going to be a lot of sleepless nights at the White House over the next few months, because the president is in a tight spot when it comes to the economy."

The supreme court is good at guarding its rulings against leaks, and so far no-one outside its tight circle has been given any indication of whether it will strike down or uphold the biggest piece of legislation of Obama's presidency, or, as with the Arizona immigration ruling earlier this week, opt for a middle way, leaving part of the legislation intact.

Romney said earlier this week he hopes the supreme court strikes down the legislation but he added that if it does not, he will make it a campaign issue, with a pledge that, if president, he would seek to repeal it.

Obama's legislation, passed in 2010 after a huge struggle to get it through Congress, opens the way for an estimated 30 million more Americans to receive coverage. The measures are not due to take effect until 2014.

The legislation has polarised opinion in the US and led directly to the creation of the Tea Party movement.

The White House spokesman, Jay Carney, speaking to reporters on Wednesday, said the White House remained confident that the Affordable Care Act, dubbed 'Obamacare' by its opponents, is constitutional. "We await the supreme court decision, as does everyone, but while we do, we continue to implement the law."

He pointed out tha individual mandate was a product of a conservative think-tank and was adopted an implemented by Romney when he was governor of Massachusetts.

Carney acknowledged that Obama opposed the individaual mandate in the presidential campaigm but was "persuaded by experts in the field that it was the best and most efficient way to ensure that we can bring the largest number of people into and under coverage".

If the supreme court rules against the individual mandate, the White House and the Democrats are likely to attempt to salvage the remainder of the legislation, in particular popular parts relating to health insurance for young people and the outlawing of insurance companies denying coverage because of pre-existing conditions.

The Democrats will also try to put the onus on the Republicans to come up with an alternative of all or part of the legislation is lost.